1. Field of the Invention
Apparatuses and methods consistent with the present invention generally relates to a direct current (DC) offset correction apparatus and method. More specifically, the present invention relates to an apparatus and method for correcting a DC offset at a receiver used in a communication system which fast hops to an ultra wide multiband.
2. Description of the Related Art
FIG. 1A is a block diagram of a conventional receiver. A local oscillator (LO) signal generated by an LO (not shown) is reflected at an antenna (not shown) or a low noise amplifier (LNA) 11, the reflected LO signal is mixed with an LO signal generated by the LO at a mixer 13, and thus a direct current (DC) offset signal is introduced. In addition, the DC offset signal may be introduced from a received radio frequency (RF) signal, in which a level of the DC offset signal is insignificant.
The DC offset signal is not removed at a low pass filter 15. Thus, the DC offset signal distorts information signals around the DC, saturates and disables an intermediate frequency amplifier (17) or an analog-to-digital converter (ADC) 19 from normal operations. Removal of the DC offset signal is required, especially the DC offset signal made at the LO signal having a relatively high signal level, so as to enhance performance of a receiver.
The following explains a conventional method to remove a DC offset signal.
FIG. 1B is a receiver using an alternating current (AC) coupling capacitor to remove a DC offset signal. The receiver of FIG. 1B includes the AC coupling capacitor 14 as compared with that of FIG. 1A. The receiver of FIG. 1B removes a DC offset signal output from a mixer 13 at the AC coupling capacitor 14 which functions as a high pass filter (HPF).
As above, the AC coupling capacitor 14 enables the removal of the DC offset signal with the most convenient and effective method. However, the AC coupling capacitor 14 attenuates information signals around DC as well as the DC offset signal, as shown in a frequency spectrum of FIG. 1C. Referring to FIG. 1C, as the DC offset signal is removed by the AC coupling capacitor 14, signals containing information also are attenuated around DC. As a result, the received information may be distorted.
Furthermore, the above DC offset compensation method using the AC coupling capacitor 14 is not suitable for a communication system which fast hops to a multiband. Since the multiband-hopping communication system uses different bands per a time interval, the receiver has to generate different LO signals for each band. In other words, the receiver needs to generate different LO signals for each time, which results in DC offset signals having different levels.
FIG. 1D depicts a level of DC offset signals caused at a receiver of a communication system that hops to three bands. In FIG. 1D, a DC offset signal with a level V1 is generated in a time interval t1 in which communications are performed across a first band, a DC offset signal with a level V2 is generated in a time interval t2 in which communications are performed across a second band, and a DC offset signal with a level V3 is generated in a time interval t3 in which communications are performed across a third band.
It takes a certain time to remove the DC offset signal at the AC coupling capacitor 14, and the certain time corresponds to a transient time of the AC coupling capacitor 14. In the case of high-speed band-hopping, another DC offset signal with a different level is input to the AC coupling capacitor 14 before the transient time of the AC coupling capacitor 14, that is, before the DC offset is completely removed. Thus, a stepped waveform is produced at each band transition time as shown in FIG. 1E. The stepped waveforms affect all bands as with a white noise and considerably deteriorates a signal to noise ratio (SNR) of the receiver.
FIGS. 2 and 3 are DC offset correction circuits that do not use an AC coupling capacitor. In detail, FIG. 2 is a DC offset correction device using a feed-forward manner, and FIG. 3 is a DC offset correction device using a feed-back manner.
In the DC offset correction device of FIG. 2, an analog signal output from a mixer 21 is converted to a digital signal by an ADC 23 and output to a DSP 25. The DSP 25 detects a level of the DC offset signal from the output signal, and generates a correction signal for the sake of the DC offset correction. The generated DC offset correction signal is converted to an analog signal at a digital-to-analog converter (DAC) 27, and the converted DC offset correction signal is added to an original signal at an adder 29. Thus, the DC offset signal can be eliminated.
In the DC offset correction device of FIG. 3, an analog signal is amplified at an amplifier 33, converted to a digital signal at an ADC 35, and input to a controller 37. The controller 37 detects a DC offset signal from the input signal, generates and outputs a DC offset correction signal to a DAC 39. The DC offset correction signal is converted to an analog signal at the DAC 39, and added to an original signal at an adder 31. As a result, the DC offset is eliminated.
As mentioned above, since a DC offset signal with a different level is generated at each time interval in the case of a multiband-hopping communication system, a DC offset correction signal different at each time interval needs to be applied to the adder 29 or 31. However, a Multi Band-Orthogonal Frequency Division Multiplexing (MB-OFDM), which is an example of the multiband-hopping system, has a band transition time of substantially 9 ns, and requires a high-speed DAC to support such a band transition time. However, it is disadvantageous to use a high-speed DAC in that the high-speed DAC is hard to implement. Moreover, even if it is possible to implement such a high-speed DAC, the high-speed DAC has a larger size than a low-speed DAC and requires a high manufacturing cost.